The present invention relates to an electrostatic copying machine comprising an improved magnetic brush developing-cleaning unit.
In a typical electrostatic copying machine, a photoconductive drum is electrostatically charged and radiated with a light image of an original document to form an electrostatic image on the drum through localized photoconduction. A magnetic brush applies a developing or toner substance to the drum to develop the electrostatic image into a toner image, which is transferred and fixed to a copy sheet to provide a permanent reproduction. Thereafter, the magnetic brush is used again to clean residual toner substance from the drum in preparation for another copying operation.
The magnetic brush comprises a non-magnetic rotary sleeve in which are coaxially disposed one or more magnets. The lower portion of the sleeve is immersed in a developing tank containing the toner substance and the upper portion of the sleeve is disposed closely adjacent to the drum. Upon relative rotation of the drum and sleeve, toner substance is magnetically attracted and adheres to the rotating sleeve to form a rotating brush thereon comprised of particles of toner substance.
In the prior art, a bias potential is applied to the magnetic brush during development which is sufficient to prevent toner substance from being attracted to the white image areas of the electrostatic image and prevent these areas from printing gray. During cleaning, the bias voltage is increased to attract all residual toner substance from the drum onto the magnetic brush. This prior art system is disadvantageous in that it does not afford sufficient control over the density of the developed toner image and does not sufficiently clean the drum. This is because the same uncontrolled amount of toner substance is present on the magnetic brush during both development and cleaning.